"Oh Shit" moments...

Sorry I got confused and meant 20feet.

Not that that change much, it's on a part of the road that turns into a hill pretty quickly.

Not exactly sure why I couldn't stop because usually I can stop pretty quickly even in the wet, was riding with a front brake.

So, riding brakeless with no foot retention at 40 mph and you couldn't even stop in 200 metres? I see what's going on there.
20 mph is about 29 feet per second, so it's no wonder you couldn't stop in 20 feet, especially on a wet road. It's all experience for you though, judging an appropriate speed is an important skill.